Exemplary embodiments generally relate to electrical testing, and more specifically, to noncontact optical testing of electrical circuits.
During semiconductor technology development and manufacturing, defects and electrical properties must be routinely monitored. For example, the most common in-line tests for middle-of-the-line (MOL) and back-end-of-the-line (BEOL) structures are for electrical open and electrical short circuits.
The most common and reliable method is electrical open and short tests where voltage is applied through a test structure utilizing test probes and current is then measured. However, testing time is becoming a bottleneck as more and more test structures (such as circuits on a wafer) need to be tested along with the increasing complexity of semiconductor technologies.
Mechanical probes for making electrical contact are inserted/pressed into the wafer surface (e.g., a chip/device under test) to electrically connect to the contacts of a circuit (the test structure) that has been fabricated on the wafer. As a result of testing electrical circuits with these test probes, significant impact on yield and reliability by probe damage has been observed in advanced technologies. For example, probe damage to copper (Cu) pads and surrounding interlayer dielectric (ILD) may cause moisture permeation and electrical open in vias (i.e., holes made from inserting probes) near test structures.